Embodiments of the present invention relate to video test patterns. More particularly, embodiments of the present invention relate to video test patterns and associated techniques for testing the fidelity of color processing.
Traditional video systems (e.g., NTSC, ATSC, etc.) transmit and store color information as luminance and chrominance signals in a YCbCr (or Y′CbCr) color space. In contrast, traditional video pickup devices (e.g., video cameras, telecines, etc.), computers, and display devices (e.g., computer monitors, televisions, etc.) capture, generate, and display color information respectively as red, green, and blue primary signals in an RGB (or R′G′B′) color space. As a result, a color video signal is converted from RGB to YCbCr at the time of capture/generation, and then converted back to RGB from YCbCr at the time of display. These first and second conversions are known as “color matrixing” and “dematrixing.”
Video output devices that are configured to perform color dematrixing (e.g., displays, computer graphics subsystems, etc.) typically have user controls labeled tint (or hue) and color (or saturation) that regulate the dematrixing process. A goal in adjusting these controls is to ensure that the video output device properly inverts the original matrixing operation, thus generating an RGB signal from a received YCbCr signal that is true to the source RGB representation.
One test pattern that has been used to aid in the adjustment of tint and color controls is known as the SMPTE (Society of Motion Picture and Television Engineers) color bars test pattern (referred to herein as “SMPTE color bars”). This pattern (illustrated in FIG. 1) includes a sequence of seven large color bars corresponding to the colors white, yellow, cyan, green, magenta, red, and blue. Directly below the seven large color bars is a sequence of smaller color bars corresponding to the colors blue, cyan, magenta, and white (interspersed with black). As shown, large white color bar 102 is adjacent to smaller blue color bar 110, large cyan color bar 104 is adjacent to smaller magenta color bar 112, large magenta color bar 106 is adjacent to smaller cyan color bar 114, and large blue color bar 108 is adjacent to smaller white color bar 116. When viewed under a condition that filters out all primaries except for blue, each pair of adjacent bars exhibit a uniform brightness, and therefore appear as a single, contiguous bar, if the tint and color controls of the display device are properly set.
While the SMPTE color bars are serviceable for calibrating tint and color controls (and thus, for testing color dematrixing accuracy), they have several shortcomings. For example, the pattern displays a limited number of side-by-side comparisons (e.g., edges) of the relevant colors of white, cyan, magenta, and blue. As shown in FIG. 1, the pattern includes only two unique edges (white/blue or cyan/magenta), and four edges total (white/blue, cyan/magenta, magenta/cyan, and blue white) between these colors. As a result, it may be difficult for users to judge the precise point at which these colors converge to a uniform brightness in the course of adjusting color and tint controls.
Another drawback is that optical blue filters (such as the Wratten 47B filter), which are commonly used in conjunction with the SMPTE color bars to block out red and green primaries, are relatively difficult and costly to manufacture. Additionally, currently available blue filters cannot completely eliminate all visibility of red and green. This reduces the accuracy of tint and color calibrations performed using the pattern.
Further, the effects of adjusting tint and color controls are interrelated, and the SMPTE color bars provide no guidance on how to adjust one control independently of the other. For example, both tint and color must be set correctly for the brightness of bars 102, 104, 106, 108, 110, 112, 114, 116 in FIG. 1 to converge. If tint is set incorrectly, the pattern provides no visual feedback on how to properly adjust color. Conversely, if color is set incorrectly, the pattern provides no visual feedback on how to properly adjust tint. Thus, users must generally resort to trial and error in finding the exact combination of tint and colors settings that result in brightness convergence of the bars.
Accordingly, what is needed are improved techniques for testing the fidelity of color processing (such as color dematrixing).